I don't think a BHS is required at L6 -- other skills could fulfill the "one acro element" requirement.
On the back tuck, some kids just are not good at standing back tucks, whether or not they are scary. DD will be L8 this year and still does not have a solid standing BT on the floor. I do not expect a solo BT to ever appear in a beam routine for her -- she is all arms and legs with a very short torso, so layouts/whips come much easier. Beyond that, a lot of girls have issues with going backwards on beam as the beam gets higher. I think there's a back walkover in the L3 floor routine, isn't there? Yet so many girls struggle so much with putting it on the beam at L5/L6!